Cleanrooms, Classes, and Control Charts

Question

My question has to do with cleanroom monitoring, specifically particle counts. Using ISO 14644-1: 1999, (we are still transitioning to the new standard), we have 27 test sites in our cleanroom. Our Particle Counter takes 10 readings per minute and averages those readings into one value. This is the number we record at each of the 27 test sites. Since an ISO Class 8 Cleanroom can have no more than 3,520,000 particles greater than 0.5 micron in size per cubic meter, for the room, do you add all 27 results and compare to the acceptance criteria? Do you average all 27 results and compare to the acceptance criteria? or as long as each individual result is below the acceptance criteria we are in compliance? what makes me think you add all results together is that the acceptance criteria states: ISO Class 8 Cleanroom can have no more than 3,520,000 particles greater than 0.5 micron in size per cubic meter, for the room. Your input would be greatly appreciated!

Answer

The way for analyzing these data is very straightforward. Considering you have continuous data, the most appropriate way for accomplishing this is by using a control chart. Use the 27 readings for constructing the averages needed for a control chart. Identify control limits and respond to special cause variation only. You will know that you are in compliance as long as your control charts remain under control. If you ever find any points out of control, you may want to quarantine the product processed during that time and conduct the appropriate corrective actions. As your regular housekeeping, keep the cleanroom certification current. The same for the particle counter system calibration.

If you are new to control charts, the easiest/simplest reference I have ever found is Understanding Variation – The Key to Managing Chaos by Donald J. Wheeler.

Based on your data performance, you may also be able to identify areas within your cleanroom that perform better than class 8 (7 or better). Knowing where those areas are may be handy.